1. Field of the Invention
The present invention relates to a lamp current balancing device, and more particularly, to a lamp current balancing device which is suitable for LCD backlight.
2. Description of the Prior Art
FIG. 1A illustrates an implementation of a prior art control circuit using a single transformer to drive double lamps. The prior art double lamp control circuit comprises a transformer 11 having a primary side connected to a electronic power switch 16 and both ends of a secondary side connected to one end of each one of two lamps 13, 14 respectively, with another end of each one of lamps 13, 14 connecting to PWM IC 15, then PWM IC 15 connecting electronic power switch 16 to form a double lamp control circuit 1; PWM IC 15 receiving and processing a feedback current signal to obtain a control signal, then outputting the control signal to electronic power switch 16 to control a pulse width outputted by electronic power switch 16, thereby controlling the brightness of lamps 13, 14. However, the above-mentioned circuit uses PWM IC 15 to provide feedback function, which cannot effectively balance the operating currents flowing through lamps 13, 14 respectively, resulting in unbalanced brightness between lamps 13, 14 and undermining the capability of lamps.
FIG. 1B illustrates an implementation of a prior art control circuit using double transformers to drive double lamps. The prior art double lamp control circuit uses two transformers 21, 22 simultaneously, each one of transformers 21, 22 having a primary side connected to a electronic power switch 26 and both ends of a secondary side connected to one end of each one of two lamps 23, 24 respectively, with another end of each one of lamps 23, 24 connecting to PWM IC 25, then PWM IC 25 connecting electronic power switch 26 to form a double lamp control circuit 2; PWM IC 25 receiving and processing a feedback current signal to obtain a control signal, then outputting the control signal to electronic power switch 26 to control a pulse width outputted by electronic power switch 16, thereby controlling the brightness of lamps 23, 24. However, the above-mentioned circuit uses PWM IC 25 to provide feedback function, which cannot effectively balance the operating currents flowing through lamps 23, 24, resulting in unbalanced brightness between lamps 13, 14 and undermining the capability of lamps. Furthermore, in the case of controlling multiple lamps, the number of transformers have to be increased proportionally, thus increasing the manufacturing cost.
FIG. 2A illustrates an implementation of a prior art control circuit using a single transformer to drive double lamps and a current transformer to balance brightness among lamps. The prior art double lamp control circuit comprises a transformer 31 having a primary side connected to a electronic power switch 36 and both ends of a secondary side of transformer 31 connected to one end of each one of two lamps 33, 34 respectively, with another end of first lamp 13 connecting to a primary side of a current transformer 37 and another end of second lamp 14 connecting to a secondary side of current transformer 37, then the primary side and the secondary side of current transformer 37 connecting to PWM IC 35, then PWM IC 35 connecting to electronic power switch 36 to form a double lamp control circuit 3. Current transformer 37 is used to control output currents of lamp 33, 34, thereby balancing the output current of lamps 33, 34. PWM IC 35 detects and processes a feedback current signal to obtain a control signal, then outputting the control signal to electronic power switch 36 to control a pulse width outputted by electronic power switch 36, thereby controlling the brightness of lamps 33, 34. Although the above mentioned circuit resolves the problem associated with using PWM IC directly as feedback mechanism, if there are big differences in specifications of lamps, bad coupling problem could arise to cause improper operation, which undermines the capability of lamps.
FIG. 2B illustrates an implementation of a prior art control circuit using double transformers to drive double lamps and a current transformer to balance brightness among lamps. The prior art double lamp control circuit comprises two transformer 41, 42, each one of transformer 41, 42 having a primary side connected to a electronic power switch 46 and both ends of a secondary side of transformer 41, 42 connected to one end of each one of two lamps 43, 44 respectively, with another end of first lamp 43 connecting to a primary side of a current transformer 47 and another end of second lamp 44 connecting to a secondary side of current transformer 47, then the primary side and the secondary side of current transformer 47 connecting to PWM IC 45, then PWM IC 45 connecting to electronic power switch 46 to form a double lamp control circuit 4. Current transformer 47 is used to control output currents of lamp 43, 44, thereby balancing the output current of lamps 43, 44. PWM IC 45 detects and processes a feedback current signal obtain a control signal, then outputting the control signal to electronic power switch 46 to control a pulse width outputted by electronic power switch 46, thereby controlling the brightness of lamps 43, 44. Although the above-mentioned circuit resolves the problem associated with using PWM IC directly as feedback mechanism, if there are big differences in specifications of lamps, bad coupling problem could arise to cause improper operation, which undermines the capability of lamps.
Therefore, the prior art lamp current control device mentioned above presents several shortcomings to be overcome.
In view of the above-described deficiency of prior-art lamp current control device, after years of constant effort in research, the inventor of this invention has consequently developed and proposed a lamp current balancing device in the present invention.